Rewrite the README sentence by sentence (closes #157)

The old text failed the issue's prose rules: a 60-word sentence on the
content security policy, a markdown table of settings, comma-chained
lists of batch sizes and error codes, undefined jargon and an edge case
in the opening. The new text leads with what RouteWatch is, why it
exists and how to run it, then the pages, the API and the blocklists,
then the settings, memory and upaas, and last the design and the
development workflow. Every statement was checked against the code on
next. The PR body lists where the old text disagreed with the code.

Model: fable-5-1

TODO.md

@@ -35,13 +35,17 @@ tested.
 
 # Completed Steps
 
+- 2026-10-27: README "Rationale" says why RouteWatch exists: to answer our other
+  services' questions about IP addresses, from its own routing table and the
+  WHOIS details it keeps. It still says what RIS Live does not give (closes
+  #204)
 - 2026-10-27: every JSON answer carries `@meta`. A panic on a JSON route answers
-  500 with the JSON error body. A method that `/status.json` or the health check
-  does not take answers 405 with it, as under `/api/v1`. Both used to answer
-  with an empty body. The home page's IP lookup shows the whole JSON answer,
-  `@meta` included. It used to show only the answer's `data`, or only the
-  message of an error answer. A test checks `@meta` on every JSON route, in its
-  successes and its errors (closes #201)
+  500 with the JSON error body. `/status.json` and the health check answer 405
+  with the JSON error body to any method they do not take. The 500 and the 405
+  used to have an empty body. The home page's IP lookup shows the whole JSON
+  answer, `@meta` included. It used to show only the answer's `data`, or only
+  the message of an error answer. A test checks `@meta` on every JSON route, in
+  its successes and its errors (closes #201)
 - 2026-10-27: every value in a JSON answer that has a unit is a number, with the
   unit at the end of its field name. `@meta.execution_time` is now
   `execution_time_ms`. The stats' `uptime`, `connection_duration` and